Unidirectional solidification is known in the production of bodies from superalloys, for example gas-turbine vanes, such that fibers, filaments or elongated crystal structures are formed in a matrix of the body by the unidirectional solidification.
The unidirectional solidification process which produces the reinforcing phase within the matrix phase sharply increases significant mechanical properties of the bodies, especially high-temperature strength and tensile strength. Improvements in other physical properties have likewise been recognized.
In general unidirectional solidification of cast alloys has been achieved heretofore by the progressive advance of a solidification front between the solidified melt and the liquid phase at a rate which permits the growth of the reinforcement phase in the form of dendrites whose principal axis extends in the direction of solidification. The rate at which the front advances is, of course, limited by the rate of solidification of the alloy and depends, in turn, upon the rate of cooling of the body of the alloy.
In prior-art processes the unidirectional solidification is generally carried out with displacement of the solidification front at a rate of about 10 - 30 cm/hr.
Because this rate is relatively low, the production of superalloy bodies by unidirectional solidification has been a relatively slow process. Various techniques have been proposed in order to modify the apparatus for unidirectional solidification so as to improve the cooling efficiency and hence increase the rate of displacement of the solidification front.
In French Pat. No. 2,184,610, for example, there is described an apparatus which permits the rate of displacement of the solidification front to reach a value of 455 cm/hr.
The increase in the rate of displacement of the solidification front has the advantage on the one hand that it reduces the time required for fabrication of a body and, on the other hand, that it permits the production of a very fine dendritic structure which has been found to be desirable for the high-temperature properties of the body.